In general, the sum of the measures of the interior angles of a quadrilateral is 360. This is true for every quadrilateral. This does not help here, because there are two angles (angles B and D) we know nothing about. We only know about opposite angles A and C.
In this case, you can use another theorem.
Opposite angles of an inscribed quadrilateral are supplementary.
m<A + m<C = 180
3x + 6 + x + 2 = 180
4x + 8 = 180
4x = 172
x = 43
m<A = 3x + 6 = 3(43) + 6 = 135
Answer: 135 deg
